Abstract
The Bull Trout Salvelinus confluentus is a char native to the Pacific Northwest with considerable ecological, cultural, and economic value that is currently listed as “Threatened” under the U.S. Endangered Species Act. Bull Trout populations are threatened by a variety of stressors, but negative interactions with nonnative Brook Trout S. fontinalis are particularly widespread. The Brook Trout is a char native to eastern North America that was widely introduced across western North America to promote sportfishing opportunities in the 19th and 20th centuries. Although Brook Trout have been recognized as a threat to Bull Trout and other native salmonids for over two decades, the ability of natural resource managers to identify and target Brook Trout where they are most harmful remains limited by several unanswered questions. Specifically, fine-scale relationships between the abundances of Brook Trout and Bull Trout remain poorly understood. Furthermore, the location and magnitude of ongoing Brook Trout invasions have been poorly documented, and colonization success has never been examined in relation to the full suite of controls suggested by ecological theory (e.g., habitat suitability, source-connectivity, biotic resistance, disturbance). Finally, the extent of Bull Trout habitats in Idaho that are suitable to (i.e., invadable by) Brook Trout is unknown, as well as the effect that climate change may have on habitat overlap among these species. In an effort to improve our ability to identify the Brook Trout populations that pose the greatest threat to Bull Trout, I sought to 1) investigate reach-scale associations between Brook Trout and Bull Trout, 2) describe ongoing Brook Trout invasions in Bull Trout streams and identify factors associated with recent colonization, and 3) develop a species distribution model for Brook Trout to identify current locations and future shifts in suitable habitat in Idaho.